New, cleaner package repository structure.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

var/spack/repos/builtin/packages/py-scikit-learn/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class PyScikitLearn(Package):
+    """"""
+    homepage = "https://pypi.python.org/pypi/scikit-learn"
+    url      = "https://pypi.python.org/packages/source/s/scikit-learn/scikit-learn-0.15.2.tar.gz"
+
+    version('0.15.2', 'd9822ad0238e17b382a3c756ea94fe0d')
+    version('0.16.1', '363ddda501e3b6b61726aa40b8dbdb7e')
+
+    extends('python')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)